\section{Integrating with Execution}
\label{sec:exec}

We have two key reasons for placing an\emph{ Over-Subscription Planner}
on-board. First, there is substantial uncertainty in the actual costs of
activities in execution. To avoid being too conservative, we want to
reserve the right to keep goals in the plan that might be feasible until
we determine otherwise. If such an event occurs, we must re-plan.
Second, we anticipate discovering new goals in-situ, potentially at
depths where surfacing the vehicle for guidance is not an option.
Consequently, making the planner available on-board is a necessity. To
accomplish this, we use T-REX [2], an Executive designed to synthesize
deliberation and execution in a seamless manner. In this case, the
planner is the deliberation algorithm for mission management. T-REX
takes care of inserting observations in the plan as execution unfolds,
dispatching the plan for execution, propagating updates in the plan to
project plan failures and invoking the planner to re-plan or to insert
newly discovered goals as the mission proceeds.
